To cope with the exponential growth in mobile data traffic, it is anticipated that new radio spectra with substantially larger bandwidths for mobile communications will be needed in the future. As a result, technologies that enable wireless communications over high-frequency bands (e.g. millimeter-wave bands) where large amounts of under-utilized spectrum are available have received much attention recently.
Communicating wirelessly over high frequency bands, such as the millimeter wave (mmW) bands, introduces some challenges. Radio signals transmitted over such bands typically suffer from higher path loss between isotropic antennas than those transmitted over the lower frequency bands that are currently used in cellular communications. The problem is further exacerbated in the unlicensed 60 GHz band where radio signals suffer additional losses due to oxygen and rain absorption, especially over long link distances.
To overcome the resulting tight link budget, wireless communications over high frequency bands often rely on a large directional gain achieved by forming narrow beams of radio signals using, for example, an adaptively steerable antenna array. Fortunately, the shortened wavelengths in high frequency bands make it possible for a device, such as an access node (AN) or user equipment (UE), of reasonable size to be equipped with a relatively large number of antenna elements for narrow beamforming. However, due to the high spatial selectivity resulting from narrow beamforming, a UE or terminal in a mmW wireless network can quickly lose connection with its serving access node (AN) due to shadowing by other moving objects.
In traditional cellular networks, the received signal quality often degrades gradually prior to the need of a handover, leaving sufficient amount of time for both the network and the user equipment (UE) to prepare for switching of serving base stations or access nodes (ANs). In mmW wireless networks, however, due to the reduced effectiveness for radio signals at high frequencies to diffract around objects and the reliance on narrow beamforming to provide adequate link SNR for high data rates, the strongest signal path can be temporarily, but abruptly, blocked by an obstacle or lost due to device rotation, causing disruptions of data flow and consequent TCP backoffs. Conventional hard handover that involves long-distance control signaling across different radio access network components may be too slow to avoid service interruption in these systems operating at high frequencies. Methods that can provide faster switching of serving ANs with minimal UE involvement in these systems are therefore desirable.